Writing a Simple DSL Compiler with Delphi. Primož Gabrijelčič / primoz.gabrijelcic.org

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1 Writing a Simple DSL Compiler with Delphi Primož Gabrijelčič / primoz.gabrijelcic.org

2 About me Primož Gabrijelčič programmer, MVP, writer, blogger, consultant, speaker Blog Skype gabr42 LinkedIn gabr42 GitHub gabr42 SO gabr Google+ Primož Gabrijelčič

3 WHY AM I HERE?

4 It all had started with a podcast

5 DSL

6 DSL? Damn Small Linux Danish Sign Language Dictionary of the Scots Language Dominican Summer League Domestic Substances List Domain Specific Language A (computer) language designed for a specific problem domain In short a programming language

7 When? When presenting a special syntax helps certain class of users Most popular DSLs: SQL, html, LaTeX, BNF, VHDL

8 VHDL Source: File:Vhdl_signed_adder_source.svg

9 FROM PROGRAM TO RESULT

10 From Program to Result Program = stream of characters Parsing Lexical analysis [lexer/tokenizer] Characters tokens Defined by regular expressions Syntactical analysis [parser] Tokens internal representation [AST] Defined by a grammar Execution Interpreter: Walk over an AST + execute step by step Cross-compiler: Walk over an AST + rewrite it as an equivalent textual output Compiler: Walk over an AST + generate machine code (for some architecture) [semantical analysis]

11 From program to result

12 Abstract Syntax Tree (AST) An abstract syntactic structure in a tree form Inessential stuff is removed Punctuation delimiters Can contain extra information position in source code Specific for a single language

13 AST Example while b 0 if a > b a := a b else b := b a return a Source: File:Abstract_syntax_tree_for_Euclidean_algorithm.svg

14 DelphiAST One unit a a time Project indexer Research project

15 GRAMMARS FOR DUMMIES

16 Grammar Set of production rules Left hand side right hand side Symbols Nonterminal [can be expanded] Terminal [stays as it is] Start Can be recursive or non-recursive Non-recursive not interesting

17 Grammar Example Example Teminals: {a,b} Nonterminals: {S, A, B} Rules: S -> AB S -> ε A -> as B -> b Language a n b n Example S -> AB -> asb -> asb -> aabb -> aabb -> aasbb -> aabb Simpler version S -> asb S -> ε Example S -> asb -> aasbb -> aabb

18 Chomsky Hierarachy Grammar Languages Automaton Production rules (constraints) Type-0 Recursively enumerable Turing machine α β (no restrictions) Type-1 Type-2 Context-sensitive Context-free Linear-bounded nondeterministic Turing machine Non-deterministic pushdown automaton Type-3 Regular Finite state automaton αaβ αγβ A γ A a A ab

19 Context-free Grammars Base of program language design Typically cannot satisfy all needs Indentation-based languages Macro- and template-based languages Attribute grammar Compiler = definition

20 Syntax vs. semantics Not all syntactically correct programs compile! Most of them don t! program Test; begin a := 1; end. Set of syntactically correct programs = CFG (possibly) Set of semantically correct programs CFG (= CSG)

21 Documenting the grammar Backus-Naur form (BNF) Extended Backus-Naur form (EBNF) Naur_form Naur_form

22 Example Pascal-like Language Source: Naur_form

23 Example Delphi 5 EBNF (partial) Source: delphi_5_grammar.html

24 PARSING

25 Parsing in Practice Lexer Typically DFA (regular expressions) Generator Custom Parser Typically LR(0), LR(1), LALR(1), LL(k) Lx top-to-bottom xl Leftmost derivation xr Rightmost derivation (n) lookahead LALR Look-Ahead LR, a special version of LR parser Generator Custom

26 LL / LR Leftmost 1. S S + S 2. S 1 3. S a Rightmost S S + S (1) 1 + S (2) 1 + S + S (1) S (2) a (3) Input: a S S + S (1) S + a (3) S + S + a (1) S a (2) a (2)

27 A SIMPLE PRIMER

28 A Simple Primer Language Addition of non-negative numbers AST Tokenizer Parser Interpreter Compiler

29 MY TOY LANGUAGE

30 My Toy Language fib(i) { if i < 3 { return 1 } else { return fib(i-2) + fib(i-1) } }

31 Specification C-style language Spacing is ignored One data type - integer Three operators: +, -, and < a < b returns 1 if a is smaller then b, 0 otherwise Two statements - if and return If statement executes then block if the test expression is not 0. Else block is required Return statement just sets a return value and doesn't interrupt the control flow There is no assignment Every function returns an integer Parameters are always passed by value A function without a return statement returns 0 A function can call other functions (or recursively itself)

32 Grammar function ::== identifier "(" [ identifier { "," identifier } ] ")" block block ::== "{" statement {";" statement} [";"] "}" statement ::== if return if ::== "if" expression block "else" block return ::== "return" expression expression ::== term term operator term term ::== numeric_constant function_call identifier operator ::== "+" "-" "< function_call ::== identifier "(" [expression { "," expression } ] ")"

33 EXTENDING THE LANGUAGE

34 Attributes AST Tokenizer Parser Interpreter Compiler fib(i) [memo] { if i < 3 { return 1 } else { return fib(i-2) + fib(i-1) } }

35 THANK YOU!

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